You can transform any ordinary rectangular doorway into a classic archway.
1. Remove any exsting door and the door jamb to expose the area back to the opening. Allow arch opening in wall frame approximately 900mm wide (between studs) and 2100mm high (from floor level). This represents an opening equivalent to a standard doorway.
2. Construct an arch template of plywood or OSB to the same sidth as the wall framing and fix template to top of opening.
3. Apply a sheet of Sheetrock plasterboard horizontally (long edges across the stud framing) to one side of the frame. With a pencil, mark the profile of the arch on the back of the sheet allowing a 13mm projection around the template. Cut the Sheetrock plasterboard around this mark using a keyhole saw.
4. Repeat this operation for the other side of the frame.
5. Measure the reveal (distance between the face sheets) and cut a strip of Sheetrock wallboard to fit between these sheets. Mix up enough cornice cement to butter both back edges of the reveal strip with 10mm beads of cement. Press the reveal strip up into the opening, hard against the template and nail it at the springing line points. Nail the face sheets to the arch template at the seven fixing points.
6. The arch template could also be used to aid in shaping the curve. Nail or staple through the Sheetrock wallboard at 300mm centres.
Finishing the Joints
1. Apply pre-mixed joint compound to a width of about 100mm on each side of the corner, filling all perforations. Use a 100mm Broad Knife. Allow 24 hours to dry. Remove any excess and lightly sand if necessary.
2. Apply a second coat to a width of about 120mm. Allow 24 hours to dry. Remove any excess again and lightly sand if necessary.
3. Apply a third coat with a 200mm broad knife. Feather the edges with a wet paint brush and allow 24 hours to dry.
4. Using sand paper and a sanding float, gently sand the dry joints to a smooth even finish. Hold the float diagonally across the joint, taking care not to scuff the paper face of the Sheetrock board where it meets the joint compound.